The invention relates to a method for supplying electrical loads of a vehicle with electrical energy. The invention further relates to an energy supply system of a vehicle, especially of a rail vehicle, for carrying out this method.
An auxiliary converter is used to supply electrical energy to an on-board network of a vehicle and to the loads connected to said network. Typical loads of a vehicle, especially of a rail vehicle, are on-board electronics for controlling/regulating the vehicle, compressors, air conditioning systems, lighting, electrical sockets etc. The loads are subdivided into a group of three-phase loads and a group of single-phase loads.
The three-phase loads are fed via a three-phase energy supply network, such as an alternating current network for example. This energy supply network comprises three phases, which are frequently referred to as L1, L2 and L3. In most cases this three-phase network does not have a neutral conductor (N conductor). However applications are also known in which the three-phase energy supply network is embodied with an N conductor. The N conductor represents the system ground. In rail vehicles in particular the three-phase energy supply network is frequently embodied as a 3AC train power supply line.
With single-phase loads the behavior is different. These are supplied with energy via a phase and a neutral conductor. Thus the single-phase energy supply network does have an N conductor. For reasons of safety this is connected at low impedance to the ground potential.
The task of the auxiliary converter is the feeding of these energy supply networks. Feeding by a number of auxiliary converters switched in parallel is also used for increasing the redundancy and thus for enhancing the fault tolerance of the energy supply.
The components absolutely necessary for the operation of the vehicle are supplied via the three-phase energy supply network. Even if in some cases an additional supply via batteries exists for these components in the event of a failure of the energy supply, the construction of the vehicle aims to design this three-phase energy supply network to be as fail-safe as possible. An especially fail-safe network represents what is known as an IT network. The IT network, also called an IT system, is a type of realization of a network, especially of a low-voltage network for electrical energy supply, with enhanced fault-tolerance in the event of short circuits to ground. In an IT network the protective grounding and the system grounding are embodied differently. The protective grounding is used for shock hazard protection of persons coming into contact with components. The system grounding is implemented by an N conductor. Where the system grounding is present at all in the three-phase energy supply network, this grounding is embodied as isolated, i.e. open. A high-impedance grounding of the N conductor is allowed in such cases. This means that in normal operation this does not result in any low-impedance connection between the phases L1, L2 and L3 of the network and the grounded parts. Thus the IT network does not possess any defined voltage potential in relation to the ground potential and is designated as potential-free. This enables the energy supply network to continue to be operated when there is a short circuit to ground in one of the phases. A warning message usually requires maintenance to be carried out in good time in the event of a detected short circuit to ground, in order to rectify the short circuit to ground. Only a further short circuit to ground in the energy supply network can lead to a failure of the energy supply network. Thus an IT network is suitable to a certain extent for the realization of a fail-safe network of a vehicle.
As well as the three-phase, fail-safe energy supply network there is often the additional requirement, especially in Europe, to operate lower-power loads that do not have such a great requirement for supply security, such as electrical sockets or lamps for example, with just a single phase (e.g. with 230V, 50 Hz). For these loads one or more additional transformers or N conductor creators are needed, which create a single-phase voltage system from the available three-phase energy supply network. This enables the three-phase energy supply network to be embodied as an IT network, i.e. isolated, and the single-phase energy supply network is embodied with grounded N conductors. Above all customers from the Asiatic and American regions often additionally require a loadable N conductor in rail vehicles, to be able to also connect single-phase loads more easily directly to a phase of the three-phase energy supply network. For reasons of safety, defined inter alia in Germany by the standard DIN VDE 0100. The N conductor is permanently grounded on the vehicle side. This type of realization is also referred to as a TT network or TT system.
The additional transformers enable an auxiliary converter to feed an energy supply network of a vehicle, which comprises both an isolated three-phase energy supply network and also a single-phase energy supply network with grounded N conductor. A short circuit to ground in the three-phase energy supply network, because of the galvanic isolation between the three-phase and single-phase energy supply network, does not lead to a failure of these two energy supply networks.
The grounding of the N conductor has the disadvantage that each short circuit to ground in the phase of the single-phase energy supply network leads to short circuit currents in the auxiliary converter. These short circuit currents can be reduced by a suitable design of the transformer, for example a high short circuit voltage uk. In addition, to protect the auxiliary converter in this case, there can be a rapid shutdown of the single-phase energy supply network, in order to protect the semiconductors of the auxiliary converter from damage.
For this reason the components absolutely necessary for the operation of the vehicle are not supplied with electrical energy from the single-phase energy supply network, but from the three-phase energy supply network, in order to make it possible to continue to operate the vehicle.